There has been conventionally known a low-pressure mercury lamp and a high-pressure mercury lamp as an ultraviolet light-emitting light source. In recent years, a small-sized and low-cost ultraviolet LED (an ultraviolet light-emitting diode) has been widely used, and it has been used for various usages such as a water sterilizer, a curing apparatus of an ultraviolet curable resin, and an ultraviolet sensor.
Conventionally, there is a quartz glass capable of efficiently transmitting ultraviolet light as a glass used for an apparatus including such an ultraviolet light source. However, there is a problem that a manufacturing cost of the quartz glass is expensive. On the other hand, there are known a phosphate glass and a borosilicate glass as a glass capable of efficiently transmitting ultraviolet light in addition to the quartz glass (for example, refer to Patent References 1(JP-A S62-027346) and 2(JP-A S60-046946)). However, these glasses have low transmittance of light with a wavelength of 400 nm or less, particularly light with wavelengths from 200 to 280 nm (hereinafter, it is also referred to as deep ultraviolet light), and improvement in the transmittance has been desired.